Radio signals may be generated or selected by means of a tuned circuit comprising inductive and capacitive elements, typically a coil and a capacitor. The combination of these elements connected in parallel or in series has a characteristic resonant frequency, that may be used to define the frequency of transmission or reception of the radio circuit. Modification of either of these components results in a change in the resonant frequency. Historically, a continuously variable capacitor has been used to tune over a particular waveband.
More recently new methods for modifying the characteristic resonant frequency of tuned circuits have been developed. In particular, the replacement of the variable capacitor with a varicap diode, also known as a varactor, enables the tuned circuit to be controlled by a voltage. The varicap diode is a reversed biased diode and by modifying the magnitude of a reverse voltage across the diode, the depletion layer, which prevents conduction, varies in thickness and acts as a variable capacitor.
In a tuned circuit that includes a varicap diode, the relationship between control voltage and resonant frequency is neither convenient nor particularly stable. Accurate control of resonant frequency is accomplished by including the tuned circuit in a radio frequency oscillator which forms part of a phase-locked loop. The output from the radio frequency oscillator may then be supplied to transmission or reception circuitry in order to define the frequency of transmitted or received radio signals.
According to a first aspect of the present invention, there is provided a method of transmitting a digital input signal over a radio channel, comprising steps of: processing said digital input signal in combination with a digital modulating signal to produce a digital intermediate signal; converting said digital signal into an analog intermediate signal; and processing said analog intermediate signal in combination with an analog modulating signal to produce a radio frequency output signal, wherein the frequency of said digital modulating signal and the frequency of said analog modulating signal are adjusted to change the frequency of said output signal.
According to a second aspect of the present invention, there is provided a method of receiving the radio frequency signal, comprising steps of: processing said signal in combination with an analog demodulating signal to produce an analog intermediate signal; sampling said analog intermediate signal to produce a digitized intermediate signal; and processing said digital intermediate signal in combination with a digital demodulating signal to select a transmission channel, wherein the frequency of said digital demodulating signal and the frequency of said analog demodulating signal are both adjusted to accommodate changes in said received signal.
Preferably, the intermediate signal is processed in combination with an additional analog demodulating signal before being sampled.
Preferably, the resolution of the analog modulating or demodulating signal is wider than a channel spacing to facilitate rapid frequency adjustments and individual channels are selected by a finer frequency resolution during the digital processing step.
According to a third aspect of the present invention there is provided apparatus for transmitting a digital input signal over a radio channel, comprising: processing means for processing said digital input signal in combination with a digital modulating signal to produce a digital intermediate signal; converting means for converting said digital intermediate signal into an analog intermediate signal; and processing means for processing said analog intermediate signal in combination with an analog modulating signal to produce a radio frequency output signal, including means arranged to adjust the frequency of said digital modulating signal and arranged to adjust the frequency of said analog modulating signal so as to change the frequency of said output signal.
According to a fourth aspect of the present invention there is provided radio frequency receiving apparatus, comprising: processing means for processing a received signal in combination with an analog demodulating signal to produce an analog intermediate signal; sampling means for sampling said analog intermediate signal to produce a digital intermediate signal; and processing means for processing said digital intermediate signal in combination with a digital demodulating signal to select a transmission channel, including means for adjusting the frequency of said analog demodulating signal and means for adjusting the frequency of said digital demodulating signal to accommodate variations in the frequency of said received signal.
In a preferred embodiment, the apparatus includes means for processing said intermediate signal in combination with an additional analog demodulating signal before said intermediate signal is sampled. Preferably, the resolution of the analog modulating signal or the analog demodulating signal is wider than a channel spacing, to facilitate rapid frequency adjustments; and individual channels are selected by a finer frequency resolution by the digital processing apparatus. Radio signals may be susceptible to Doppler shifts and Doppler shift compensation may be provided by adjusting the frequency of the digital modulating or digital demodulating signals in the digital signal processing domain.